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          shell命名之fg
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        <h4 id="基本的几个指令"><a href="#基本的几个指令" class="headerlink" title="基本的几个指令"></a>基本的几个指令</h4><h5 id="amp"><a href="#amp" class="headerlink" title="&amp;"></a><code>&amp;</code></h5><p>这个用在一个命令的最后，可以把这个命令放到后台执行</p>
<p><code>ctrl + z</code></p>
<p>可以将一个正在前台执行的命令放到后台，并且暂停</p>
<p><code>ctrl + d</code></p>
<p>表示结束当前输入(即用户不再给当前程序发出指令)。</p>
<p>它不是发送信号，而是表示一个特殊的二进制值，表示EOF。</p>
<span id="more"></span>
<p><code>jobs</code></p>
<p>查看当前有多少在后台运行的命令</p>
<p><code>fg</code></p>
<p>将后台中的命令调到前台继续运行</p>
<p>如果后台中有多个命令，可以用<code>fg %jobNumber</code>将选中的命令调出，<code>%jobNumber</code>是通过<code>jobs</code>命令查到的后台正在执行的命令的序号(<strong>不是pid</strong>)</p>
<p><code>bg</code></p>
<p>将一个在后台暂停的命令，变成继续执行</p>
<p>如果后台中有多个命令，可以用<code>bg %jobNumber</code>将选中的命令调出，<code>%jobNumber</code>是通过<code>jobs</code>命令查到的后台正在执行的命令的序号(<strong>不是pid</strong>)</p>
<h4 id="延申：nohup"><a href="#延申：nohup" class="headerlink" title="延申：nohup"></a>延申：<code>nohup</code></h4><h5 id="一个栗子"><a href="#一个栗子" class="headerlink" title="一个栗子"></a>一个栗子</h5><figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_">$ </span><span class="language-bash"><span class="built_in">nohup</span> /root/start.sh &amp;</span></span><br><span class="line"><span class="meta prompt_"># </span><span class="language-bash">原程序的标准输出被自动改向到当前目录下的nohup.out文件，起到了<span class="built_in">log</span>的作用。</span></span><br></pre></td></tr></table></figure>
<h5 id="nohup命令"><a href="#nohup命令" class="headerlink" title="nohup命令"></a>nohup命令</h5><p>用途：不挂断的运行命令。</p>
<p>语法：<code>nohup Command [arg...] [&amp;]</code></p>
<blockquote>
<p>描述：nohup 命令运行由 Command 参数和任何相关的 Arg 参数指定的命令，忽略所有挂断（SIGHUP）信号。在注销后使用 nohup 命令运行后台中的程序。要运行后台中的 nohup 命令，添加 &amp; （ 表示”and”的符号）到命令的尾部。</p>
</blockquote>
<h5 id="nohup后面的2-gt-amp-1的含义"><a href="#nohup后面的2-gt-amp-1的含义" class="headerlink" title="nohup后面的2&gt;&amp;1的含义"></a>nohup后面的<code>2&gt;&amp;1</code>的含义</h5><h6 id="基本符号与含义"><a href="#基本符号与含义" class="headerlink" title="基本符号与含义"></a>基本符号与含义</h6><ul>
<li><code>0</code>表示stdin标准输入</li>
<li><code>1</code>表示stdout标准输出</li>
<li><code>2</code>表示stderr标准错误</li>
</ul>
<h6 id="command-gt-file"><a href="#command-gt-file" class="headerlink" title="command &gt; file"></a><code>command &gt; file</code></h6><p>这个命令其实是一个缩写，实际上是command产生的标准输出重定向到file中，也就是说相当于执行了<code>command 1 &gt; file</code>。</p>
<h6 id="2-gt-amp-1"><a href="#2-gt-amp-1" class="headerlink" title="2&gt;&amp;1"></a><code>2&gt;&amp;1</code></h6><p>2是标准错误，1是标准输出，&amp;的意思是 <strong>等效于</strong>。</p>
<p>实际上就是把标准错误也重定向到file中</p>
<h6 id="command-gt-a-2-gt-amp-1与command-gt-a-2-gt-a的区别"><a href="#command-gt-a-2-gt-amp-1与command-gt-a-2-gt-a的区别" class="headerlink" title="command&gt;a 2&gt;&amp;1与command&gt;a 2&gt;a的区别"></a>command&gt;a 2&gt;&amp;1与command&gt;a 2&gt;a的区别</h6><blockquote>
<p>经过上面的分析，<code>command&gt;a 2&gt;&amp;1</code>这条命令，等价于<code>command 1&gt;a 2&gt;&amp;1</code>，也就是说执行command产生的标准输入重定向到文件a中，标准错误也重定向到文件a中，那么是否等价于<code>command 1&gt;a 2&gt;a</code>呢？其实不是，区别在于前者只打开一次文件a，后者会打开两次并导致标准输出被标准错误<strong>覆盖</strong>。</p>
<p><code>&amp;1</code>的含义就可以理解为用标准输出的引用，引用的就是重定向标准输出产生打开的a。从IO效率上来讲，<code>command 1&gt;a 2&gt;&amp;1</code>比<code>command 1&gt;a 2&gt;a</code>的效率更高。</p>
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